Wi-Fi Technical Posts-What is MIMO? Highlighted

Hello,

Today I would like to share with you what is MIMO?

We know that Wi-Fi signals are sent and received through antennas, what should I do if the Wi-Fi rate is not enough? Then add a few more antennas to transmit data. This technology that uses multiple antennas to transmit wireless signals to improve communication quality is MIMO. The IEEE 802.11n protocol introduced MIMO technology for the first time, and the theoretical Wi-Fi rate has been greatly improved.

Definition of MIMO

According to the number of antennas at the transceiver, wireless communication can be divided into four types: SISO, SIMO, MISO, and MIMO.

Their meaning is as follows:

• SISO: Single Input Single Output (Single Input Single Output)

• SIMO: Single Input Multiple Output (Single Input Multiple Output)

• MISO: Multiple Input Single Output (Multiple Input Single Output)

• MIMO: Multiple Input Multiple Output (Multiple Input Multiple Output)

MIMO is an antenna system composed of m transmitting antennas and n receiving antennas. The signal is transmitted and received through multiple antennas at the transmitting end and the receiving end, thereby improving communication quality. MIMO achieves multiple transmissions and multiple receptions through multiple antennas. Without increasing the spectrum resources and antenna transmission power, it can double the system channel capacity, showing obvious advantages, and is regarded as the core technology of the next generation of mobile communications.

Broadly speaking, SIMO and MISO also belong to the category of MIMO.

How does MIMO transmit data

MIMO has multiple transmission paths, and data transmission is not simple. In order to understand how MIMO transmits data, we need to understand the concepts of diversity and multiplexing in wireless communication.

• Diversity

Diversity is to repeatedly send a piece of data multiple times to ensure that the receiving end can receive it correctly and improve the reliability of transmission.

In the SISO system, there is only one transmission path at the transmitter and receiver, and we can send the same data at different times. This method is called time diversity.

When there are multiple antennas on the receiving or transmitting end, we can use different transmission paths between the transmitting and receiving antennas to send the same data. This method is called spatial diversity.

• Reuse

Diversity improves the reliability of data transmission, but the transmission rate is not high. Because the content is the same, only one piece of data is actually transmitted at a time. When there are multiple antennas at the receiving or transmitting end, we can multiplex different transmission paths in space and send multiple copies of different data in parallel to increase capacity. This method is called space division multiplexing.

The SISO system sends and receives single data, and the data transmission is relatively simple. Let's start with the SIMO system and see how the data is diversified or multiplexed.

• SIMO

In the SIMO system, the transmitter can reach the receiver through two paths, but because the transmitter has only one antenna, only the same data X can be sent on two paths at the same time. This method is also called receive diversity. Although the transmission rate remains the same, the probability that the receiving end can correctly receive the data has increased.

• MISO

In the MISO system, the two antennas at the transmitting end can send the same data X to the receiving end at the same time. This method is called transmit diversity, which improves the reliability of data transmission. Can the two antennas at the transmitter send different data to increase the transmission rate? That is the space division multiplexing mentioned above, let's analyze this problem below.

Assuming that the two antennas at the transmitting end send X and Y respectively, the channel gains of transmission path 1 and path 2 are h1 and h2, respectively. Considering coherent demodulation, that is, h1 and h2 are known at the receiving end, then the signal received by the receiving antenna is h1 ·X+h2·Y, we can easily find that there is only one equation, and the receiving antenna cannot solve X and Y. Therefore, in the MISO system, the antenna at the transmitting end can only send the same signal at the same time.

• MIMO

In the MISO system, the antenna at the transmitting end can only transmit the same signal at the same time. What about MIMO? Let’s take 2×2 MIMO as an example. Because the receiving end has an additional antenna, the signal received by the receiving end’s antenna is h1·X+h2·Y, h3·X+h4·Y. Because of the addition of an equation, the receiving end’s The antenna can easily solve X and Y, so in 2×2 MIMO, it has the conditions for transmitting and receiving two channels of data at the same time for space division multiplexing.

However, it should be noted here that the maximum transmission capacity of the transceiver depends on the number of antennas on both sides of the transceiver, and is subject to the one with fewer antennas. For example, there are 4 antennas at the transmitting end and only 2 antennas at the receiving end, so the antennas at the transmitting end can only send 2 channels of different data at the same time.

In the actual communication process, the sender and receiver will adaptively adjust and choose the best way to communicate based on factors such as the instant communication conditions and the propagation environment. For example, when the wireless channel conditions are poor, diversity technology will be used more to ensure the reliability of communication; when the channel conditions are good, multiplexing will be selected, and more data will be sent each time to improve transmission. rate.

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